Objectives and Background: Individuals with Phelan–McDermid Syndrome (PMS) present with a variety of symptoms, including a breadth of behavioral issues. Clinically assessing behavior in PMS remains challenging due to the overabundance of behavioral assessments and the lack of tools validated explicitly for use in individuals with intellectual disability (ID) and neurodevelopmental disorders. This review sought to suggest which assessment tools would best clinically assess behavior in individuals with PMS. Methods: Validated behavioral assessment tools were identified using a systematic search of the literature, and relevant data for each assessment were extracted. The consensus guidelines for PMS were reviewed. Results: This review identified 131 validated assessment tools that were categorized by the intended age group and into specific behavioral domains: Autism spectrum disorder (ASD) screening, adaptive behavior, restricted and repetitive behaviors, challenging/disruptive behaviors, mental health screening, and other miscellaneous behaviors such as avoidance and impulsivity. Discussion: Based on the 2023 consensus guidelines, suggestions were given on which tools would be best for assessing various symptoms and behaviors in PMS. Choosing the best assessment tools to appraise behavior and related symptoms in individuals with PMS will aid clinicians in decision-making and lead to more personalized treatment plans.